There are only a small handful of volumes available as of yet -- Grundrisse, economic notebooks from 1863-67, and volume II of Capital -- but this is the material condition for a wave of new, high-quality Marx scholarship. The bound volumes of the MEGA are insanely expensive, and full collections (not that MEGA is yet complete by any measure) are only available at a very few libraries in the world. Most people who want to do serious work on Marx are forced to rely upon the old and inadequate Marx-Engels-Werke or the English-language Complete Works.

I was able to use the MEGA for my dissertation work, luckily, and it made a huge difference. The mass of Marx's lifework was left in manuscript or notebook form when he died, and the editions of this material that eventually emerged did not do a very good job of carrying over the actual form and history of the manuscripts.

Even the published works suffer from this editorial erasure of history. The most egregious example I know of is this: if you are working on Capital I, and, in due diligence, look at the German Werke version, you find a book divided into 7 Abschnitten and 25 Kapiteln. But all of the English translations are divided into 8 Parts and 33 Chapters. You might be led thereby to think that this difference is some artifact of the translation, or of Engels' postmortem editorial tampering. You'd be wrong. For completely mysterious reasons, the MEW folks took the text of the 4th German edition of the book (1890), but divided it according to the 2nd German edition (1873). The 8 Parts/33 Chapters arrangement came on the scene with the French edition of the book (1875, the last edition overseen to completion by Marx himself), and was incorporated, on Marx's instructions, into the 3rd German edition (1883), and then into the English edition (1886). Why the editors of the Werke decided to undo this authorial decision is beyond me. Only by looking at MEGA, which published each of these editions of the book as separate volumes, can you see what happened.

I have no reason to doubt that there are many, many, many other examples of this sort of thing. The more widely available the MEGA becomes, the easier it will be for students of Marx to discover and correct these sorts of errors.